The molecular structures of the polyamideimide resins vary depending on the monomers employed. But typical examples thereof are those represented by the following formula (I) ##STR1## prepared by polycondensation of trimellitic acid (or its derivatives) as an aromatic tricarboxylic acid component with m-phenylene diamine and diaminodiphenyl ether as an aromatic distains component.
Polyamideimide resins of the formula (I) are disclosed in U.S. Pat. No. 4,045,407 and Japanese Patent Publication HEI 02-18,422. They are transparent and noncrystalline resins which have the following properties:
(1) They have a heat distortion temperature of 278.degree. C. and a long term heat resistance temperature exceeding 200.degree. C. They have excellent heat resistance. They are usable in the broad temperature range of -200.degree. C. to 260.degree. C.; PA0 (2) They not only have high mechanical strength but also good stiffness providing physical property at room temperature of general engineering plastics even at temperatures exceeding 200.degree. C. They also have excellent impact resistance. PA0 (3 ) They also have creep resistance; PA0 (4) They have a small linear expansion coefficient of 4.times.10.sup.-5 cm/cm..degree.C., which can be reduced to less than half by using filler. PA0 (5) They have excellent insulation breakdown strength and volume resistivity, and show flame retardance of UL 94 V-O without adding additives. PA0 (6) They have good wear resistance property due to their composition of PTFE and graphite. They are suitable as sliding member under severe circumstance since they have good self-lubricating properties, wear resistance and strength and elasticity even at high temperature. PA0 (7) They have good chemical resistance and are stable in hydrocarbon solvents, through care must be taken in concentrated aqueous alkali solution. PA0 (8) They have good ultraviolet resistance and radiation resistance.
Examples of the methods for preparing a polyamideimide resins generally include the isocyanate method and acid chloride method.
The isocyanate method comprises condensating aromatic diisocyanate with aromatic tricarboxylic acid anhydride to give polyamideimide without through polyamic acid which is intermediate polymer as disclosed in Japanese Patent Laid-Open Publication No. SHO 48-19274 and U.S. Pat. No. 3,541,038(1970).
The acid chloride method comprises the condensation of aromatic tricarboxylic acid chloride with aromatic diamine. This method is classified into "low temperature homogeneous solution polymerization method" and "low temperature precipitating polymerization method". The typical example of the low temperature homogeneous solution polymerization method comprises the polymerization reaction at room temperature in nonaqueous type polar solvent such as N,N-dimethylacetamide (hereinafter referred to as DMAC) which was developed by Standard-Oil-Co., in the U.S.A. as disclosed in U.S. Pat. No. 3,920,612 (1975). The typical example of the low temperature precipitating polymerization method comprises the polymerization reaction in an organic solvent which is sparingly soluble in water, such as methyl ethyl ketone (for example, produced by Teijin Kasei Corp. in Japan) and in an aqueous solvent by using triethylamine as an acid acceptor as disclosed in Japanese Patent Laid-Open No. SHO 46-15,513. This reaction is a sort of interface polymerization method.
Another method of preparing polyamideimide resin is the direct polymerization method which comprises direct polymerization of aromatic diamine with aromatic tricarboxylic acid in the presence of dehydration catalyst as disclosed in U.S. Pat. No. 3,860,559 (1975) and Japanese Patent Publication No. SHO 58-180532.
However, the isocyanate method has problems in that gelation occurs during the reaction and it is difficult to give linear polymers having high molecular weights due to the formation of side reaction products. Therefore, the polyamideimide resins prepared by the method have decreased melt flowability, poor mold processability, mechanical properties and heat resistance, and thus are not suitable for application as molding materials.
By the low temperature homogeneous solution polymerization method, polymers having high molecular weights can be obtained by using very expensive acid chloride as raw materials. However, this makes the cost for raw material very high. Furthermore, since this method is carried out in two steps consisting of preparing polyamic acid as a primary polymer and then imidization of the latter by heating or by using dehydration catalyst and it is required to remove halogen compound which is formed during the reaction, the method is economically less interesting. This method also has a problem in that the modification of the molecular structure of the resins is almost impossible.
The low temperature precipitating polymerization method is carried out in two steps as low temperature homogeneous polymerization method, which consists of precipitating polyamic acid by using very expensive acid chloride and using methyl ethyl ketone and water as a reaction solvent and subsequently subjecting it to ring closing treatment. The polyamideimide resins prepared by the two steps have a large molecular weight distribution and low molecular weights. Thus, the method has no practical use.
Both the isocyanate method and acid chloride method have disadvantages in that the handling of acid chloride and diisocyanate is troublesome since both acid chloride and diisocyanate are sensitive to water, which should be blocked in the reaction process.
Meanwhile, the direct polymerization method of polyamideimide resins comprises directly polymerizing the aromatic diamine with aromatic tricarboxylic acid anhydride (or its derivatives) in the presence of polymerization catalyst. The advantages of this polymerization method are that the process of this method is simplest among many preparing processes, the cost for raw materials and processing is not high, and that the handling of monomers is easy. For these reasons active researches have been made in this field. The examples of polymerization catalyst used in the method are phosphoric acid types such as phosphoric acid and polyphosphoric acid (Japanese Patent Publication Nos. SHO 63-27,527 and SHO 62-297,329, boric acid types such as boric acid and boric acid anhydride (French Patent No.1,515,066 and Japanese Patent Publication No.58,180,532) or triphenyl phosphite and phosphoric triester type (U.S. Pat. No. 3,860,559). The effect of these polymerization catalysts depends on the type of catalyst. However, to obtain polymers having high molecular weights by using costly polymerization catalyst and reactant monomers in a same molar ratio, it is necessary to carry out the reaction at high temperatures of 200.degree. C. or more for a lengthy period. Thus, even if in the case of using a high boiling point solvent such as N-methyl pyrrolidone (hereinafter referred to as NMP), sulfolan and nitrobenzene as a synthesizing solvent, tar-state substance formed by decomposition of monomers and generated resins in reaction vessel and polymerization catalyst used in a large amount are incorporated into the polymer, which cause unsatisfactory color and the decrease of physical properties of polyamideimide. Moreover, this process is disadvantageous in that it is difficult to give linear polymers having high molecular weights due to side reaction and thereby the solubility of polymers is decreased. Particularly, this process has no economical merit since high costly polymerization catalyst has to be used in a large quantity and the recovery of this catalyst is impossible.
Based on the problems of the above-mentioned processes, the present invention have made extensive studies in order to solve the problems of the known direct polymerization method and to find a process for preparing polyamideimide resins having high molecular weights with good heat resistance and melt flowability. As a result, the present inventors have now found that by dissolving an aromatic tricarboxylic acid anhydride (or its derivatives) and an aromatic diamine in polar solvent and subsequently polymerizing the resultant solution in the presence of polymerization catalyst, polyamideimide resins having high molecular weights can be produced. The present invention has been attained on the basis of this finding.